In recent years, attention has been given to CDMA communication system that is potent against interference and jamming as a communication system used for mobile communication system.
CDMA communication system is the one wherein user signals, which are desired to transmit, are spread with spreading codes to transmit the user signals on a transmitting side, while inverse spreading is made by the use of the same spreading codes as that described above on its receiving side thereby to acquire the original user signals.
Furthermore, in CDMA communication system multipath components due to phasing and the like are synthesized, whereby reliability in data is improved.
In the following, a constitution of a mobile station in CDMA communication system will be described.
Data is transmitted from a base station to a mobile station. In this occasion, however, there is a case where a plurality of paths exists depending upon an environment of the mobile station. For instance, where there are obstacles such as buildings, trees and the like in an environment of the mobile station, radio waves are reflected by these obstacles to present the plurality of paths.
FIG. 1 shows an example of a constitution of a mobile station in a conventional CDMA communication system wherein three paths (radio waves A, B, and C) reside in between a base station 20 and the mobile station 30.
As shown in FIG. 1. the mobile station 30 in the present constitutional example is composed of an antenna 1, an RF section 2, an AD section 3, a delay profile section 4, a finger allocating section 8, a plurality of finger sections 9, and a rake synthesizing section 7.
The antenna 1 receives a synthesized wave composed of a plurality of radio waves (radio waves A, B, and C) being arrived from the base station 20 through the plurality of paths. The RF section 2 converts the synthesized wave received by the antenna 1 into analog base band signals. The AD section 3 converts the analog base band signals converted in the RF section 2 into digital base band signals.
The delay profile section 4 acquires data transmitted from the base station 20 by spreading inversely with the use of the digital base band signals converted in the AD section 3. In this case a delay profile is prepared by adding cumulatively the data acquired in a certain time interval for retrieving multipath components. In the case where multipath components exist, a plurality of peaks of the radio waves (radio waves A, B, and C) can be detected in a delay profile as shown in FIG. 2.
The finger allocating section 8 allocates path timings (reference timings) corresponding to positions of the respective peaks in the plurality of the radio waves detected in the delay profile section 4 to a separate finger sections 9, respectively.
Each of the finger sections 9 spreads inversely the digital base band signals converted by the AD section 3 at the respective path timings allocated by the finger allocating section 8, whereby data transmitted from the base station 20 is regenerated. The rake synthesizing section 7 synthesizes the data regenerated in the respective finger sections 9 to output demodulated data.
In the following, operation for path detection in the delay profile section 4 will be described.
The delay profile section 4 adds cumulatively data transmitted from the base station 20 for a certain period of time to prepare a delay profile. Such cumulative addition is implemented for the sake of discriminating a plurality of peaks in radio waves from noises, and in this respect, the longer period of time for cumulative addition can improve the better reliability in peak points.
However, too long period of time for cumulative addition brings about a possibility of displacement in peak points due to out of alignment in reference timing in between the base station 20 and the mobile station 30, influence of clock jitter inside the mobile station 30 or the like as shown in FIG. 3.
Accordingly, each of the finger sections 9 tracks a path at a shorter cycle than a period of time for cumulative addition in the delay profile section 4 in order to follow positional changes (displacement) in peak points of the path.
In the following, path tracking operation in each of the finger sections 9 will be described by referring to FIG. 4.
FIG. 4 is a block diagram showing an example of a constitution of each of the finger sections 9 shown in FIG. 6.
As shown in FIG. 4, the finger section 9 in the present constitutional example is composed of an inversely spreading section 11, an electric power computational section 12, a maximum value detecting section 13, and a selector section 14.
The inversely spreading section 11 converts digital base band signals converted by the AD section 3 into data. Furthermore, the inversely spreading section 11 performs inverse spreading within a predetermined segment positioned between before and after segments of a path timing allocated by the finger allocating section a (hereinafter referred to as “path tracking range”). For instance, when inverse spreading is performed within path tracking ranges defined among five sections extending over before and after path timing, the inversely spreading section 11 outputs symbol data reside among the five sections, i.e., five symbol data.
The electric computational section 12 computes each of electric power values of the five symbol data output from the inversely spreading section 11. The maximum value detecting section 13 detects the maximum value among five electric power values computed by the electric power computational section 12.
The selector section 14 selects only the symbol data having the maximum electric power value detected by the maximum value detecting section 13 among the five symbol data computed in the inversely spreading section 11 to output them.
As described above, inverse spreading is implemented within a predetermined path tracking range among segments positioned in between before and after the path timing instructed by the finger allocating section 8 thereby to acquire data, and the maximum electric power value is retrieved from the data, so that the finger section 9 follows delicate fluctuation.
In a conventional mobile station as mentioned above, however, when it is assumed that there are three radio waves being arrived from a base station to the mobile station (radio waves A, B, and C) and that each of path timings in the radio waves A, B, and C is allocated to each of separate finger sections in the case where distances of peak positions in the three radio waves are narrow, respectively, as shown in FIG. 5, inverse spreading is implemented within a predetermined path tracking range defined among segments positioned in between before and after a path timing in each finger section. As a result, inverse spreading timings in all the finger sections overlap with the radio wave A residing at a point of the maximum electric power value, whereby a plurality of radio waves comes to be not received, so that there is a problem of deteriorating reception property in mobile station.